1. Field of the Invention
The present invention relates to a laser diode, and more particularly, to a buried ridge waveguide laser diode having a current block layer.
2. Description of the Related Art
Optical waveguide structures of laser diodes can be generally divided into gain guiding types and index guiding types according to the principle of forming a guiding beam in a lateral direction. The index guiding types can further be divided into strongly index guiding types and weakly index guiding types according to the structure of the diode. The strongly index guiding type has a structure in which an active layer that generates an optical gain is formed to have a limited width in the lateral direction so that an optical gain and an optical guide can be generated in a particular active layer. A buried hetero-structure is a representative structure of the strongly index guiding type. The weakly index guiding type diode has a structure in which identical active layers are formed in the lateral direction and an additional structure that changes refractive index is included on or below the active layer. The weakly index guiding type diode indirectly guides an optical beam using the additional structure, and can be of two types: a ridge type and a rib type.
The weakly index guiding type waveguide diode of the buried ridge type can be formed by one growing and etching process. Accordingly, the weakly index guiding type waveguide diode is easy to manufacture, and has uniform characteristics, high reliability. Also, since the weakly index guiding type waveguide diode has a low electrostatic capacity, it can be operated at high speed.
However, the weakly index guiding type has a large threshold current when compared to the strongly index guiding type due to lateral carrier spreading or diffusion, and the realization of lateral direction single mode characteristics is difficult since the width of the guide is narrow. To obtain the lateral direction single mode characteristics, the width of the ridge must be manufactured as narrow as possible. However, since it is difficult to deposit an electrode using a photolithography etching process after a window is opened, manufacturing a ridge weakly index guiding-laser diode (RWG-LD) having 3 μm or less is difficult. Also, when the ridge region is formed, there is a step difference between the ridge region and regions outside the ridge region. To remove the step difference, a thick metal layer must be formed on the regions outside the ridge region by plating a metal or a polyimide material. The maximum width of the ridge to obtain the lateral direction single mode characteristics, although it can be slightly increased by reducing the thickness of the active layer, is known to be approximately 5 μm. If the width of the ridge is greater than the above limit, a lateral multi-mode occurs, and eventually, the utilization of the diode is difficult due to a kink phenomenon.
To solve the problems of the weakly index guiding structure, that is, the ridge type laser diode, a new structure has been proposed in Journal of the Optical Society of Korea vol. 12, no. 4, pp 312-319, 2001. Also, this article discloses the results of optimization of a weakly index guiding structure. FIG. 1 is a cross-sectional view of the structure of a ridge type waveguide laser diode disclosed in the above article.
Referring to FIG. 1, the laser diode has a structure in which regions outside the ridge region (peripheral region) are filled with indium phosphate (InP) having a relatively high refractive index (refractive index of InP=3.17), and the lateral direction refractive index is controlled according to the thickness of InGaAsP 24 additionally grown in the ridge region. For reference, in a conventional ridge type waveguide laser diode, the regions outside the ridge region are filled with a material having small refractive index such as air (refractive index=1) or polyimide (refractive index=1.8). The regions outside the ridge region are filled with an n-type current blocking layer 22. Therefore, a current is injected only into the ridge region. This laser diode is a weakly index guiding structure since the active layer is not limited in the lateral direction and the optical beam is guided according to the structure of the InGaAsP formed on the active layer. Also, this diode is referred to as a buried ridge waveguide laser diode since the ridge region is buried in the current blocking layer 22.
In the above structure, the lateral direction refractive index can be controlled by controlling the thickness of the InGaAsP. Therefore, the lateral direction single mode characteristics can be obtained in a relatively wide ridge width, for example, 6 to 9 μm. Also, since the regions outside the ridge region are filled with a current blocking layer, the conventional polyimide process or a metal plating process is unnecessary. According to the structure and a design method proposed in the above article, the operation of the lateral direction single mode is possible when the width of the ridge region is approximately 7 μm.
In forming a lateral direction single mode, the buried ridge waveguide laser diode can be operated with a relatively larger ridge width when compared to a conventional ridge type waveguide laser diode. Also, when the ridge region of the buried ridge type waveguide laser diode is formed, it can be readily formed since the conventional polyimide process or the metal plating process is unnecessary.
In the buried ridge type waveguide laser diode, a current flows only into a limited region, that is, the ridge region due to the n-p InP layer (the n-p current blocking layer) in the regions outside the ridge region. However, the current density in the ridge region is increased since the current is injected only into the limited region. The increased current density increases the thermal resistance and series resistance in the ridge region, thereby reducing the light emission characteristics, particularly, the temperature characteristics. Also, the optical loss increases due to a leakage current through a bonding area between the n-p InP layer and the p-InP layer.